Scanners may be used to scan an image to create a scanned image which can be displayed on a computer monitor, which can be used by a computer program, which can be printed, which can be faxed, etc. One conventional method for scanning an image uses a scanner having a subscan axis, a scan bar having sensor elements (such as CCD [charge-coupled-device] elements), and a scan-bar shading calibration strip having a white area and optionally a black area.
It is noted that each optical sensing element may produce a signal proportional to the amount of light reaching the element. The proportion or “gain” of each element may be related but not necessarily identical. In addition, the light source may not uniformly illuminate the document to be scanned. To get an image with a consistent representation, the elements should be individually calibrated (also referred to as “shaded”) using a calibration strip with a white area and optionally a black area.
To perform shading, the scan bar, including the sensor elements, may be moved along the subscan axis over the white area (and optionally over the black area) of the shading calibration strip, and reference values of the output signal of the sensor elements may be obtained. The white-area reference values (and optionally also the black-area reference values) for a particular sensor element may be used to calibrate that sensor element. In one known method, the average of all of the white-area reference values for a particular sensor element is used to calibrate the particular sensor element to the white area of the shading calibration strip. Calibration may provide a revised gain for each CCD element to compensate for varying amounts of illumination produced by a scanner light source in different regions of a scanned image and to compensate for variations among the CCD elements of the scan bar. However, optical defects, occlusions or blemishes, such as dust, etc., on the shading calibration strip may cause the calibration of sensor elements which pass over the optical defects to be inaccurate. When printing a scanned document, inaccurate calibration of a CCD element may result in a vertical shading artifact, wherein a printed vertical column may be brighter or darker than neighboring columns. In another known method, the white-area reference values for a particular sensor element may be arranged in a histogram, and values outside a predetermined limit may be considered to be from blemishes on the shading calibration strip and are not included in taking an average of the reference values, wherein the average is used for calibrating the particular sensor element.
It is known to apply an edge detection filter to detect the edges of scanned text in optical character recognition applications, wherein the edges are then filtered to present a sharper image.
What is needed is an improved method for shading an optical sensing element such as an optical sensing element of a scanner.